Interactions of Deep-Water Gravity Flows and Active Salt Tectonics

Abstract

Sediment gravity flow behaviour is influenced by seafloor topography associated with salt structures, which controls the depositional architecture of deep-water sedimentary systems. Typically, salt-influenced deep-water successions are poorly-imaged in seismic reflection data and exhumed systems are rare, hence the detailed sedimentology and stratigraphic architecture of these systems remains poorly understood. The exhumed Triassic (Keuper) Bakio and Guernica salt bodies in the Basque-Cantabrian Basin, Spain were active during deep-water sedimentation. The salt diapirs grew reactively, then passively, during the Aptian-Albian, and are flanked by deep-water carbonate (Aptian-earliest Albian Urgonian Group) and siliciclastic (middle Albian-Cenomanian Black Flysch Group) successions. The study compares the deposition in two salt-influenced confined (Sollube basin) and partially-confined (Jata basin) minibasins by actively growing salt diapirs, comparable to salt-influenced minibasins in the subsurface. The presence of a well-exposed halokinetic sequence, beds that pinch out towards topography, soft sediment deformation, variable paleocurrents and intercalated mass transport deposits (MTDs) indicate that salt grew during deposition. Overall, the Black Flysch Group coarsens- and thickens-upwards in response to regional axial progradation, which is modulated by laterally-derived MTDs from halokinetic slopes. The variation in type and number of MTDs within the Sollube and Jata basins indicate the basins had different tectono-stratigraphic histories despite their proximity. In the Sollube basin, the routeing systems were confined between the two salt structures eventually depositing amalgamated sandstones in the basin’s axis. Different facies and architectures are observed in the Jata basin due to partial confinement. The findings show exposed minibasins are individualised and that facies vary both spatially and temporally in agreement with subsurface salt-influenced basins. Salt-related, active topography and the degree of confinement are shown to be important modifiers of depositional systems, resulting in facies variability, remobilisation of deposits and ‘channelisation’ of flows. The findings are directly applicable to the exploration and development of subsurface energy reservoirs in salt basins globally, enabling better prediction of depositional architecture in areas where seismic imaging is challenging.

Halokinetic modulation of sedimentary thickness and architecture: A numerical modelling approach

“…Siliciclastic depositional environments show a thinning of sandstone towards the topographic high and an overall concentration of high reservoir quality units at the base of topography (Figure 12a,d), such that a salt‐related combined structural‐stratigraphic trapping mechanism becomes unlikely (Figure 13; Kane et al., 2012; Stricker et al., 2018). Muddier (lower reservoir quality) and thinner (lower net‐to‐gross) units are expected closer to the diapir (Figures 8, 10e and 11; Banham & Mountney, 2013a; Cumberpatch, Kane, et al., 2021). These units are more likely to be over‐pressured due to upward rotation, creating a large pressure head, with the top seal rocks unable to hold back a significant hydrocarbon column (Figure 13; Heidari et al., 2017, 2019; Nikolinakou et al., 2014a, 2014b, 2018).…”

“…(b) Location map of the field analogue, for a full geological discussion, see Cumberpatch, Kane, et al. (2021). (c) Outcropping Triassic evaporites on Bakio Beach, believed to be part of the Bakio Diapir.…”

“…Exposed Aptian‐Cenomanian strata adjacent to the Bakio Diapir, northern Spain document an increase in sedimentation rate associated with an increase in erosion of the hinterland (Figure 12; García‐Mondéjar, 1996; Martín‐Chivelet et al., 2002; Puelles et al., 2014), similar to M5 increasing from ‘slow’ to ‘fast’ relative sedimentation rates. The resulting progradational pattern manifests in the deep‐water succession as an upward change from thin‐bedded low‐density turbidites, deposited in the lobe fringe, (equivalent to S1) to thick‐bedded high‐density turbidites deposited in a lobe axis (equivalent to S3) (Cumberpatch, Kane, et al., 2021). Early diapiric (the Urgonian Group: Figure 12) and syn‐kinematic (the Black Flysch Group) depositional elements are deformed closest to the diapir, and deformation intensity decreases away from the diapir, being minimal outside the halokinetically influenced sequence (ca.…”

“…Supplementing subsurface data with modelled stratal architectures and depositional facies observations from exhumed halokinetically influenced settings globally (e.g. Banham & Mountney, 2013a, 2013b, 2014; Counts & Amos, 2016; Counts et al., 2019; Cumberpatch, Kane, et al., 2021; Poprawski et al., 2014; 2016; Ribes et al., 2015, 2017) is recommended as a useful workflow for building reservoir models for salt basins with limited data. Observations from multiple scales and types of models can be combined to further reduce the uncertainty associated with reservoir quality prediction, for example, recent FEM has shown porosity is lower than expected near the vertical parts of salt structures and higher than expected at the base of diapirs, due to mean principal stress variations (Nikolinakou et al., 2014a).…”

“…Giles & Lawton, 2002; Giles & Rowan, 2012), deep‐ (e.g. Cumberpatch, Kane, et al., 2021; Poprawski et al., 2014, 2016) and non‐marine stratigraphy (e.g. Banham & Mountney, 2013a, 2013b, 2014; Ribes et al., 2015, 2017).…”